The subject matter of this specification relates generally to search systems.
The World Wide Web (the “web”) contains a vast amount of information. A user can use a search engine to find the information on the Web in which the user is interested. The user enters a search query into a search engine. The search engine matches the search query to an index of crawled web pages. The web pages in the index that satisfy the search query are returned to the user as hyperlinks. The web pages that satisfy the search query can be ranked by the search engine based on various factors, including, for example, the relevance of the web page to the search query.